It is known that it is desirable to produce elastomeric polymers capable of exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents and then vulcanized. Such elastomers, when fabricated into components for constructing articles such as tires, vibration isolators, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomer refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in lowered fuel consumption of vehicles using such tires and prolonged tire life. In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system.
Various synthetic strategies have been developed to provide elastomers with molecular structures exhibiting reduced hysteresis energy losses. One technique is to produce elastomers of very high molecular weight. In such high molecular weight systems, the number of free uncrosslinked molecular chain-ends per given weight in the vulcanizates made from them are reduced. Since the presence of free, unbound chain ends are believed to be a significant factor in hysteretic energy loss because they cannot participate in elastic recovery processes, their reduction leads to a desirable reduction in hysteretic energy loss.
Another technique is to prepare elastomer molecules with end groups capable of interacting with the reinforcing fillers such as carbon black present in compounded elastomer compositions. Again, such interaction reduces the number of free end groups believed to contribute to hysteretic losses. Such interactive end groups include those derived from various metal reagents as well as those derived from polar organic reagents such amines, amides, esters, imines, imidies, ketones and various combinations of such groups. Heretofore, it has not been known to produce modified elastomers with purely hydrocarbyl terminal functionality which is capable of conferring low hysteresis properties. For example, commonly assigned U.S. patent application Ser. No. 07/636,961 describes elastomers with tin containing end-groups derived by initiating polymerization under anionic conditions with tin-lithium compounds such as trialkyl tin (IV) lithium, that is, (alkyl).sub.3 SnLi groups. This application does not disclose or suggest reaction of polymer with modifiers that produce non-polar (that is, non-hetro atom-containing) end groups. Thus the elastomer-modifying reactions occurring in the present invention are inherently different than those described in the above-noted reference.
Another approach to elastomers with reduced hysteresis properties of elastomer compounds involves "jumping" of elastomer intermediates having terminal functionality that is reactive under anionic polymerization conditions. Such jumping reactions join two elastomer molecules to produce a single molecule of much higher molecule weight. In the present invention the modified elastomers contain vinyl arene functionality which can readily participate in jumping reactions. This functionality can also participate in intermolecular network formation during the vulcanization process. Such participation again reduces the number of free, unbound chain ends in the vulcanizate which results in low hysteretic energy loss.
The present invention is directed to (halomethyl vinyl arene)-modified elastomers which, when compounded and vulcanized by known rubber processing techniques, provide vulcanized elastomers which exhibit desirable low hysteresis characteristics. These (halomethyl vinyl arene)-modified elastomers can be made by reacting lithium-terminated elastomer intermediates with more than about 0.8 equivalents of at least one (halomethyl vinyl) arene. The modified elastomers thereby produced contain vinyl arene functionality which can either interact with vulcanizate fillers such as carbon black, participate in covulcanization network formation or serve as sites for molecular weight-increasing reactions. Mixtures of (halomethyl vinyl arene)-modified elastomers with unmodified elastomers derived from the lithium-terminated elastomer intermediates are also useful as are jumped and covulcanizate compositions made from them.